P692: IMMUNE FACTORS MAINTAINING TREATMENT-FREE REMISSION IN PATIENTS WITH CHRONIC MYELOID LEUKEMIA AFTER DISCONTINUATION OF IMATINIB
Authors: P. Kwaśnik, J. Zaleska, D. Link-Lenczowska, M. Zawada, B. Ochrem, G. Bober, E. Wasilewska, E. Mędraś, I. Hus, M. Szarejko, W. Prejzner, O. Grzybowska-Izydorczyk, A. Klonowska-Szymczyk, T. Sacha, K. Giannopoulos
Journal: HemaSphere (2022)

Background: CML treatment aims to achieve long-term remission without treatment (TFR). The mechanisms that are responsible for CML relapses remain elusive. It is suggested that maintaining TFR is not directly related to the total disposing of the gene transcript BCR-ABL1, but it might be a result of the immune surveillance restoration in CML. Aims: To determine immune factors responsible for elimination or control over residual CML cells, which may be crucial for achieving long-term relapse-free survival (RFS) and TFR after imatinib discontinuation. Methods: Peripheral blood mononuclear cells from 63 CML patients were analyzed using flow cytometer analysis at two-time points: at discontinuation of imatinib and 3 months after withdrawal. The major populations of the immune system that have a potential effect on the maintenance of the patient in remission were assessed. BCR-ABL1 transcript were analyzed using two molecular biology methods: standard real-time quantitative polymerase chain reaction (RQ-PCR) and droplet digital PCR (ddPCR). Results: We found a significant increase in the percentage of HLA-DR+ cells (median 1.25 vs 1.90%, p<0.0001), mDC (median 0.20 vs 0.28%, p<0.01), pDC (median 0.10 vs 0.15%, p<0.01) and a significant decrease in PD-1 expression on mDC cells (median 35.81 vs 32.23%, p=0.03) in 3 months after treatment discontinuation. There was a significantly reduced level of NKT cells (median 14.45 vs 12.80%, p=0.02), a significantly increased PD-1 expression on CD161+ cells (median 21.02 vs 21.28%, p=0.02) and a significantly decrease of PD-1 expression on CD56brightCD16- cells (median 2.31 vs 2.08%, p=0.04). Analyses of T cells showed a significant decrease in frequencies of CD4+CD25+ T cells (median 18.01% vs 13.42%, p<0.0001), CD4+CD25+FOXP3+ (median 3.00 vs 2.89%, p=0.02) and a significantly lower expression levels of PD-1 on CD4+ cells (median 21.59 vs 19.34%, p=0.02) after 3 months compared to moment of withdrawal. We found a weak negative correlation between age and CD19+ cell depletion (p<0.01, R=-0.39) and a moderate positive correlation between age and PD-1 expression on CD19+ cells (p<0.001, R=0.43) at the moment of withdrawal. In addition, we found a weak positive correlation between age and HLA-DR+ cell count (p=0.01, R=0.32) and a weak negative correlation with CD19+ cell count (p=0.04, R=-0.26) in 3 months after stopping treatment. Significantly higher levels of NK cells: CD56dimCD16+ (median 8.93 vs 14.23%, p<0.01) and CD56brightCD16- (median 0.33 vs 0.52%, p=0.02) were found in men compared to women 3 months after withdrawal. Analysis of the BCR-ABL1 transcript by both RQ-PCR and ddPCR methods in relation to the immune subpopulations showed significant differences between the percentage of HLA-DR+ cells and the depth of the molecular response: in RQ-PCR greater than or equal to MR4.0 at the moment of withdrawal vs loss of MR4.0 after 3 months (median 1.24 vs 1.87, p=0.03); greater than or equal to MR4.0 at the moment of withdrawal vs greater than or equal to MR4.0 after 3 months (median 1.24 vs 1.92, p<0.001). Summary/Conclusion: The demonstrated changes in the percentage of populations of immune cells show the importance of immunocompetent cells’ involvement in maintaining the patient in TFR. The characterization of the immune system, which probably plays the most important role in achieving long-term relapse-free response and maintaining remission without treatment, may help define the group of patients who could safely discontinue imatinib. Funding: NCN 2018/31/B/NZ6/03361